70 STRUCTURAL BOTANY 



corpuscles, but they are less numerous here, and are 

 distributed uniformly in the protoplasm lining the 

 cell- walls. This tissue is called the spongy parenchyma. 

 The lower ends of the palisade cells never end blindly 

 in an intercellular space, but always join on to cells of 

 the spongy parenchyma. 



The finer vascular bundles pass through the upper 

 part of the spongy tissue in such a position that their 

 xylem almost reaches the lower ends of the palisade 

 cells. 



The differentiation of the rnesophyll into palisade 

 and spongy tissue is characteristic of those flat leaves 

 which are horizontal, and which therefore have an 

 upper and a lower surface. Both tissues take part in 

 carbon-assimilation, the palisade - parenchyma being 

 specially adapted to the intense light which falls on 

 the upper surface of the leaf. The spongy parenchyma 

 is actively concerned in the transpiration of watery 

 vapour (see Chap. III., p. 220). 



The epidermis of the upper surface of the leaf 

 consists entirely of closely-packed cells, without inter- 

 cellular spaces. It bears a few long, spindle-shaped 

 hairs, like those of the stem. There are no stomata 

 on this upper surface. 



The epidermis of the lower surface has more 

 numerous hairs, and possesses an immense number 

 of stomata (see Fig. 27). The general structure of 

 a stoma has already been described. Each stoma 

 opens into a large intercellular space in the spongy 

 tissue immediately within it. This space is called 

 the air-chamber (see Fig. 26). The air-chambers, 

 again, are in communication with all the intercellular 

 spaces of the leaf, and through them, with those of the 

 whole plant. The stomata, then, are the pores by 



